In order to increase the efficiency and decrease the manufacturing cost of photovoltaic (PV) cells, significant efforts have been made to develop rear-contact solar cells in which both the positive and negative polarity contacts of the solar cell are accessible from the rear, or non-light-incident, side of the cell. Compared to traditional front-contact solar cells, rear-contact solar cells typically have less or in some cases no metal coverage of the cell's front surface. This circumvents the tradeoff that occurs in front-contact cells between the conductance of metallic front electrodes and their coverage (or shadowing) of the cell's light-incident side, leading to better optical in-coupling, lower resistive power loss, and higher conversion efficiency. Examples of rear-contact solar cells may be found in U.S. Pat. Nos. 3,903,427, 3,903,428, 4,927,770, 5,053,083, 7,276,724, and US patent applications US2009/0314346, US2010/0139746, and US2009/0256254. A thorough review of silicon-based rear-contact solar cell technology may be found in Prog. Photovolt: Res. Appl. 2006; 14:107-123.
In addition to providing higher efficiency, there are at least two other ways in which the incorporation of rear-contact solar cells can simplify and reduce the cost of manufacturing PV modules. First, in a rear-contact PV module production line it may be possible to replace the tabbing and stringing operation required in front-contact PV modules with a simple placement step in which rear-contact cells are directly connected to an electrically-functional “conductive backsheet” just prior to module lamination. This can help enhance the overall throughput of the production line. Second, for silicon-based PV cells, rear-contact PV modules are typically better-suited to the incorporation of thin, large cells than front-contact PV modules because front-contact silicon cells develop a large coefficient of thermal expansion (CTE) mismatch stress when thick current-collecting tabs are soldered to them. This CTE mismatch stress and associated cell breakage is particularly problematic if the cells are thinner than about 200 microns or larger than about 156 millimeters on a side. By contrast, the need for thick metallic conductors is significantly reduced in rear-contact solar cells because the output current is typically distributed across the back surface of the cell. This enables thinner, wider metallic conductors to be attached to the back of rear-contact solar cells with low ohmic power loss and reduced breakage from CTE mismatch stress.
At present, however, several factors related to the difficulty in interconnecting rear-contact solar cells have limited their widespread implementation. One factor arises from the challenging dimensional requirements involved in the interconnection process. In many rear-contact solar cell architectures it is desirable to have a contact spacing on the order of a few millimeters on the cell's rear surface, while the interconnected assembly of rear-contact solar cells is generally 1 m2 or larger in a finished PV module. It is difficult to fabricate a single circuit or device that can accommodate both of these dimensional requirements with high yield and low cost. Prior art large-area conductive backsheets typically utilize a “flex circuit” process in which a layer of conductive foil is patterned into interdigitated positive and negative electrodes using mask and etch techniques (see, for example, US Patent App. No. 2010/0012172). In many cases the production cost of these conductive backsheets is so high that their use in PV modules becomes impractical. The high cost can be attributed in part to the relative lack of availability of screen printing and etching equipment that can handle 0.5-2 m wide rolls of material, and in part to the low throughput associated with etching thick metal foils.
In addition, achieving sufficient long-term reliability from rear-contact PV modules incorporating large-area conductive backsheets has been a challenge. For example, these devices may be prone to electrical shorting during fabrication and/or long-term outdoor exposure if an electrode of one polarity on the conductive backsheet touches an electrode of the opposite polarity on a rear-contact solar cell. In addition, although silicon solar cells are less likely to break during the module assembly of rear-contact PV modules than front-contact PV modules, in some cases CTE mismatch effects (e.g., between silicon solar cells and metallic foil conductors) in rear-contact PV modules can lead to the buildup of significant mechanical stress during day/night temperature cycling of the module. Over the long term this can lead to wrinkling and/or delamination of the conductive foil layer from the rear-contact solar cells, or, if the effect is severe enough, to the mechanical failure of the materials such as solder or electrically conductive adhesive (ECA) that are used to make electrical connections between the solar cells and conductive backsheet. This can potentially result in a significant reduction in power output from the rear-contact PV module.